1. Field of the Invention
The present invention relates to an integrated circuit for controlling charging of a secondary battery, and more particularly to an integrated circuit for determining whether a secondary battery is set in a charging device on the basis of the charging state of the secondary battery. In addition, the present invention also relates to a charging device using the integrated circuit, and a method for determining whether a secondary battery is set in a charging device.
2. Discussion of the Background
Until now, the following methods have been used for determining whether a secondary battery is set in a charging device.    (1) methods using a switch which achieves an ON state when a secondary battery is set in a charging device while achieving an OFF state when no secondary battery is set (disclosed in, for example, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 10-7285812); and    (2) methods in which a current is applied to a temperature detection thermistor, which is included in a battery pack, to check the voltage of the terminal of the thermistor, resulting in determination whether or not a battery is present (disclosed in, for example, Japanese patent No. (hereinafter referred to as JP) 2927354 (i.e., JP-A 01-186126).
The first-mentioned methods have a drawback in that a switch has to be provided, thereby increasing the size and costs of the device. The second-mentioned methods have drawbacks in that the methods cannot be used for batteries having no thermistor; and it is necessary to connect a terminal with the thermistor, thereby increasing the size and costs of the device. In addition, although the first-mentioned and second-mentioned methods can determine whether a battery is set in a charging device but cannot determine whether a battery is electrically connected with a terminal of the charging device.
In addition, the following methods, which do not use a switch or a thermistor, have been proposed (for example, JP 3203538 (i.e., JP-A 08-168188):    (3) methods in which a charge current is applied to a battery when a voltage not less than a predetermined voltage is detected between terminals of a charging device connected with the battery, wherein when the charge current is greater than a predetermined threshold current in a full charge state, it is judged that the battery is connected, and when the charge current is less than the threshold current, it is judged that the battery is fully charged. In addition, two additional threshold currents are set near the above-mentioned threshold current. When the time needed for changing the current from one of the additional threshold currents to the other is not less than a predetermined time, it is judged that the battery is fully charged. Further, when the time is less than the predetermined time, it is judged that the battery is detached from the charging device.
Further, the following methods, which do not use a switch or a thermistor, have been proposed (for example, JP-A 10-225001):    (4) methods which perform intermittent charging while measuring the voltage between terminals connected with a battery and the charge current in a charging process, wherein whether or not a battery is connected is determined on the basis of the results.
However, the third-mentioned methods have a drawback in that an over-discharge battery cannot be charged. Specifically, when charging is performed on an over-discharge battery while applying a large current thereto, a serious accident such as heat generation of heat or fire may occur, and therefore it is necessary to charge the battery while applying a small current until the battery has a predetermined voltage. Therefore, the threshold voltage which is used for judging whether or not a battery is present has to be set so as to be greater than the voltage of an over-discharge battery. In addition, the method in which two additional threshold currents are set near the above-mentioned threshold current (i.e., full-charge current) to determine whether a battery is connected and the battery is fully charged has a drawback in that the scale of the circuit increases.
It is necessary for the fourth-mentioned methods to perform intermittent charging in addition to general constant current-constant voltage charging, and therefore an intermittent charging device has to be provided. Accordingly, the methods have a drawback in that the size and costs of the charging device increase.
Because of these reasons, a need exists for an integrated circuit (and a charging device), which can determine whether a secondary battery is correctly connected with a charging device while having a small size and low costs.